A Solid Oxide Fuel Cell (SOFC) comprises a solid electrolyte that enables the conduction of oxygen ions, a cathode where oxygen is reduced to oxygen ions and an anode where hydrogen is oxidised. The overall reaction in a SOFC is that hydrogen and oxygen electrochemically react to produce electricity, heat and water. In order to produce the required hydrogen, the anode normally possesses catalytic activity for the steam reforming of hydrocarbons, particularly natural gas, whereby hydrogen, carbon dioxide and carbon monoxide are generated. Steam reforming of methane, the main component of natural gas, can be described by the following equations:CH4+H20→CO+3H2 CH4+CO2→2CO+2H2 CO+H20→CO2+H2 
During operation an oxidant such as air is supplied to the solid oxide fuel cell in the cathode region. Fuel such as hydrogen is supplied in the anode region of the fuel cell. Alternatively, a hydrocarbon fuel such as methane is supplied in the anode region, where it is converted to hydrogen and carbon oxides by the above reactions. Hydrogen passes through the porous anode and reacts at the anode/electrolyte interface with oxygen ions generated on the cathode side that have diffused through the electrolyte. Oxygen ions are created in the cathode side with an input of electrons from the external electrical circuit of the cell.
To increase voltage, several cell units are assembled to form a stack and are linked together by interconnects. Interconnects serve as a gas barrier to separate the anode (fuel) and cathode (air/oxygen) sides of adjacent cell units, and at the same time they enable current conduction between the adjacent cells, i.e. between an anode of one cell with a surplus of electrons and a cathode of a neighbouring cell needing electrons for the reduction process. Further, interconnects are normally provided with a plurality of flow paths for the passage of fuel gas on one side of the interconnect and oxidant gas on the opposite side. To optimize the performance of a SOFC stack, a range of positive values should be maximized without unacceptable consequence on another range of related negative values which should be minimized. Some of these values are:
VALUES TO BE MAXIMIZEDVALUES TO BE MINIMIZEDFuel utilizationPriceelectrical efficiencyDimensionslife time(temperature, to a point)production timefail ratenumber of componentsParasitic loss (heating,cooling, blowers . . . )
Almost all the above listed values are interrelated, which means that altering one value will impact other values. Some relations between the characteristics of gas flow in the fuel cells and the above values are mentioned here:
Fuel Utilization:
The flow paths on the fuel side of the interconnect should be designed to seek an equal amount of fuel to each cell in a stack, i.e. there should be no flow- “short-cuts” through the fuel side of the stack.
Parasitic Loss:
Design of the process gas flow paths in the SOFC stack and its fuel cell units should seek to achieve a low pressure loss per flow volume at least on the air side and potentially on the fuel side of the interconnect, which will reduce the parasitic loss to blowers.
Electric Efficiency:
The interconnect leads current between the anode and the cathode layer of neighbouring cells. Hence, to reduce internal resistance, the electrically conducting contact points (hereafter merely called “contact points”) of the interconnect should be designed to establish good electrically contact to the electrodes (anode and cathode) and the contact points should no where be far apart, which would force the current to run through a longer distance of the electrode with resulting higher internal resistance.
Lifetime:
Depends in relation to the interconnect, on even flow distribution on both fuel and air side of the interconnect, few components and even protective coating on the materials among others.
Price:
The interconnects price contribution can be reduced by not using noble materials, by reducing the production time of the interconnect and minimizing the material loss.
Dimensions:
The overall dimensions of a fuel stack is reduced, when the interconnect design ensures a high utilization of the active cell area. Dead-areas with low fuel- or air flow should be reduced and inactive zones for sealing surfaces should be minimized.
Temperature:
The temperature should be high enough to ensure catalytic reaction in the cell, yet low enough to avoid accelerated degradation of the cell components. The interconnect should therefore contribute to an even temperature distribution giving a high average temperature without exceeding the maximum temperature.
Production Time.
Production time of the interconnect itself should be minimized and the interconnect design should also contribute to a fast assembling of the entire stack. In general, for every component the interconnect design renders unnecessary, there is a gain in production time.
Fail Rate.
The interconnect production methods and materials should permit a low interconnect fail rate (such as unwanted holes in the interconnect gas barrier, uneven material thickness or characteristics). Further the fail-rate of the assembled cell stack can be reduced when the interconnect design reduces the total number of components to be assembled and reduces the length of seal surfaces.
Number of Components.
Apart from minimizing errors and assembling time as already mentioned, a reduction of the number of components leads to a reduced price.
The way the anode and cathode gas flows are distributed in a SOFC stack is by having a common manifold for each of the two process gasses. The manifolds can either be internal or external. The manifolds supply process gasses to the individual layers in the SOFC stack by the means of channels to each layer. The channels are normally situated in one layer of the repeating elements which are comprised in the SOFC stack, i.e. in the spacers or in the interconnect.
Spacers or interconnects normally have one inlet channel which is stamped, cut or etched all the way through the material. The reason for only having one inlet channel is that the spacer has to be an integral component. This solution allows for a cheap and controllable manufacturing of the spacer or interconnect channel, because controllable dimensions give controllable pressure drops.
Another way of making process gas channels, which allows for multi channels, is by etching, coining, pressing or in other ways making a channel partly through the spacer or interconnect. This means that the spacer can be an integral component, but the method of making the channels partly through the material is not precise, which gives an uncertain and uncontrollable pressure-drop in the gas channels.
If a sealing material is applied across gas channels which are formed only partly through the material of the spacer or the interconnect, more uncertain and uncontrollable pressure-drops in the gas channels will arise. The sealing material can of course be screen printed to match only the desired surfaces, or glued and cut away from the gas channels, which will lower the risk of uncertain pressure-drops, but this is expensive and time-consuming.
U.S. Pat. No. 6,492,053 discloses a fuel cell stack including an interconnect and a spacer. Both, the interconnect and the spacer, have inlet and outlet manifolds for the flow of oxygen/fuel. The inlet and outlet manifolds have grooves/passages on its surface for the distribution of oxygen/fuel along the anode and cathode. However, the grooves/passages of the interconnect and spacer are not aligned with each other and hence their geometries could not be combined to achieve multiple inlet points. Also, since the grooves/passages are on the surface of both the interconnect and spacers, the formation of multiple inlet points are not feasible.
US2010297535 discloses a bipolar plate of a fuel cell with flow channels. The flow plate has multiple channels for distributing fluid uniformly between the active area of the fuel cell. The document does not describe a second layer and similar channels within it.
US2005016729 discloses a ceramic fuel cell(s) which is supported in a heat conductive interconnect plate, and a plurality of plates form a conductive heater named a stack. Connecting a plurality of stacks forms a stick of fuel cells. By connecting a plurality of sticks end to end, a string of fuel cells is formed. The length of the string can be one thousand feet or more, sized to penetrate an underground resource layer, for example of oil. A pre-heater brings the string to an operating temperature exceeding 700 DEG C., and then the fuel cells maintain that temperature via a plurality of conduits feeding the fuel cells fuel and an oxidant, and transferring exhaust gases to a planetary surface. A manifold can be used between the string and the planetary surface to continue the plurality of conduits and act as a heat exchanger between exhaust gases and oxidants/fuel.
None of the above described known art provides a simple, efficient and fail-safe solution to the above described problems.
Therefore, with reference to the above listed considerations, there is a need for a robust, simple, cheap and easy to produce and handle, multi-channel gas inlet solution to provide an efficient and fail minimizing gas inlet for an SOFC unit. As corresponding cell units can also be used for solid oxide electrolysis, this gas inlet solution can also be used for a SOEC unit, hence a solution is sought for a SOC unit.
These and other objects are achieved by the invention as described below.